1. Field of the Invention
The present invention relates to a thin-film semiconductor device such as a thin-film transistor (referred to as TFT hereinafter) made up of a semiconductor thin-film formed on an insulating substrate, a manufacturing method thereof, and an image display apparatus using such a device.
2. Description of the Related Art
In recent years, as the amount of information in electronic data form increases, the development of apparatus which processes and visually displays such information is becoming more and more important. With an increase in the size of image display apparatus and image sensors along with growth of the demand for higher integration density (higher precision) of pixels, the use of TFTs capable of offering high-speed drivability is required. To satisfy these requirements, it is inevitably required to develop advanced technology which makes it possible to fabricate, at low costs, TFTs made of high-quality silicon (Si) thin-films on or above a low cost electrically insulating substrate, such as a large-size glass substrate or the like.
Conventionally, methods for crystallizing an amorphous Si thin-film are known as high-quality Si thin-film fabrication technology. These methods in turn involve a laser-aided crystallization technique, which has been widely employed until recently. For example, a Si thin-film crystallized by use of an excimer laser is a polycrystalline silicon thin-film with its average grain size ranging from about 0.1 to 1.0 μm. In the case of fabrication of a TFT of the metal oxide semiconductor (MOS) type, a crystal grain boundary inevitably exists within the channel region of such a TFT. This results in a decrease in carrier mobility, which in turn leads to a degradation of performance.
Another problem faced with the approach is as follows. During fusion crystallization, a difference in cubical or volume expansion coefficients between a liquid Si and solid Si results in unwanted creation of surface convex-concave irregularity at the grain boundary, causing the TFT to decrease in withstanding or breakdown voltage. In view of these problems, a technique for enabling Si crystals to increase in grain diameter while at the same time offering surface planarization capabilities is strongly required.
As an example of a method of improving the performance of TFTs, an apparatus is disclosed, for example, JP-A-11-121753, which apparatus permits crystals to grow to have an increased length in a specified direction while letting a source/drain layout direction (equivalent to a current flow direction) become almost identical to the elongate direction of crystal grains thus grown. Additionally, in a liquid crystal display device which is disclosed for example in JP-A-2000-243970 as an embodiment, the layout directions of the sources and drains of the TFTs are arranged to almost coincide with the elongate direction of the crystal grains. Each TFT is disposed to have a longitudinal/lateral block shape (in a horizontal direction and in a vertical direction) at a display pixel array peripheral portion, when viewed from the surface side of an array substrate. However, since any one of the TFTs is such that its channel region is not single-crystallized, the performance and reliability decrease by the influence of a trap level that exists at grain boundaries. This, in turn, causes a problem that the characteristics increase in variability.
Recently, crystallization technologies have been widely used which employ solid-state lasers (such as YAG laser or the like) extremely higher in beam stability than excimer lasers. However, while rectangular single-crystal grains are formed in a laser scan direction, the average width thereof is merely from about 0.5 to 1.5 μm. Thus, it has been impossible to eliminate unwanted creation of grain boundaries within TFT active regions. To be brief, the known approaches all suffer from the presence of a plurality of grain boundaries within the active regions of the TFTs and also encounter a problem that the grain boundary number in each TFT active region can vary. As a result, the TFT characteristics are undesirably varied accordingly.
It is apparent from the technical problems stated above that in the fabrication technique for forming a high-quality polycrystalline film on a dielectric film by laser annealing methods, it remains difficult to form TFTs of high performance due to the lack of regularities of crystal grain size and face orientation, and also due to difficulties in crystal grain position control and others. Accordingly, in order to fabricate, by low-cost manufacturing methods, TFTs that are high in performance and reliability and yet low in variation, it is required to single-crystallize by relatively simple methods at least the active regions of the TFTs on an insulating substrate.